Rhesus monkeys were injected intravenously with tritium-labeled testosterone (T), progesterone (P), 17α-hydroxyprogesterone (17 OHP), norethynodrel (N) and mestranol (M) to determine whether these steroids can enter the cerebrospinal fluid (CSF). These studies showed that all these steroids can enter the CSF. The concentrations of T and P in the plasma were consider-ably more than those found in the CSF, whereas the levels of 17 OHP, M and N were considerably more in the CSF than in the plasma. The possible significance of this difference is discussed.